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Simulation of a De Laval Nozzle

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Old   June 3, 2015, 02:17
Default Simulation of a De Laval Nozzle
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Hello there!

I am pretty new to this forum and to ANSYS CFX, and I've been trying to simulate the flow of air through a De Laval Nozzle with a subsonic inlet and a supersonic outlet. I have scrummaged through some of the threads on the De Laval Nozzle here, but I couldn't really find anything on what I am having trouble with. Anyway, these are my inlet and outlet parameters:

Fluid Model : Total Energy with Shear Stress Transport
Inlet : Subsonic
Mass flow at the inlet : 0.69 kg/s (This is the only inlet parameter that is available)
Material : Air at 650 degree C (I am not sure if I have to enter this as the value of the Static Temperature or the Total Temperature at the inlet, since there is a dialog box for Static/Total Temperature, or whether I should enter this at all)
Outlet : Supersonic (Velocity > 550 m/s)

I tried setting the outlet flow type to 'Supersonic' with a monitor point near the outler region, but when I do the solution, I seem to get really wrong velocity values like 0.5-0.6 m/s at the monitor point. I don't really know what I am doing wrong here.

This is the nozzle I have sketched.
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Old   June 3, 2015, 06:20
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CFX ask you the temperature of your Inlet since you have enter a energy equation (total energy whit shear stress transport). If all your walls are adiabatic (usual for turbomachinery), and there is no big change of energy you can disable it.

But here since you are transonic, i will keep the energy equation (this is only my point of view).

what is the boundary condition you use for the outlet (is it velocity outlet ?)

Is the material air with constant density or air ideal gas with inlet at 650C ?

if it is velocity, then the solver doesn't have a pressure reference.

the basic couple of inlet/outlet conditions are :
Total Pressure / Pressure
Velocity / Pressure
MassFlow / Pressure

Moreover I personally avoid using massFlow of Velocity in a blocked nozzle because you are at the point the massFlow is limited at the col (M=1).

you can approximate the theoretical maximal massFlow of a nozzle by this formula :

Q = AreaCol x PstatAval x (gamma / R / TempAval)^0.5 x (2 / (gamma + 1))^((gamma + 1)/2/(gamma - 1))

(sorry for the long formula)
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